In the process of manufacturing semiconductor products such as a semiconductor integrated circuit, a liquid crystal panel, a solar panel or a magnetic disk, there has been used an apparatus for generating plasma in a noble gas atmosphere in order to carry out various processes for semiconductor device by the plasma.
In this plasma process, the noble gas is introduced into a process chamber in a nitrogen atmosphere in order to generate plasma by high-frequency discharge, and when the processed product is taken out, the chamber is purged by charging nitrogen.
Although argon was conventionally used as the noble gas to be employed in such treatments, recently, krypton and xenon are coming to the fore for more sophisticated applications.
However, krypton and xenon are extremely expensive, since the existence ratio of them in air is very small and the process of separating them from air is complicated. Therefore, there has been a problem that the cost for krypton and xenon rises dramatically if these gases are used as the atmosphere gas for the process and exhausted (wasted) to outside.
In order to economically establish the process using these noble gases, it is important to recover the noble gas from the exhaust gas at a high recovery rate and reuse it.
In order to reuse the noble gas, it is necessary to separate noble gas with an impurity concentration of 100 ppm or less from the exhaust gas.
As a method of separating a desired component from a mixed gas containing a plurality of components, a pressure swing adsorption method is available.
This pressure swing adsorption method is applicable to a method of recovering noble gas from the above mentioned exhaust gas.
In a conventional pressure swing adsorption method, when a readily adsorbable component is adsorbed onto an adsorbent during an adsorption step, a less readily adsorbable component is also slightly adsorbed at the same time.
Furthermore, since the less readily adsorbable component remains in the interparticle void space in an adsorption column, an exhaust gas containing a large amount of the less readily adsorbable component is exhausted to outside the system during a regeneration step. Therefore, it is difficult to recover the desired gas at high recovery rate (less readily adsorbable component in this example).
In order to recover the desired component from the mixed gas at a high recovery rate, it is indispensable to maintain the amount of the desired component contained in the gas exhausted to outside the system at a minimum.
As conventional pressure swing adsorption methods, there are a rate-dependent pressure swing adsorption method, which uses the difference in adsorption rates of adsorbed gases, and an equilibrium pressure swing adsorption method, which uses the difference in the amounts of adsorbed gases at equilibrium. It is possible to improve the recovery rate by combining these methods.
In case combining these two separation methods, for example, the gases discharged during the regeneration steps of the rate-dependent pressure swing adsorption process and the equilibrium pressure swing adsorption process are all recovered in a feed gas storage tank, mixed with a feed gas, and recirculated during the adsorption steps as a circulating feed gas.
In this method, since the gas discharged during the regeneration step are supplied to adsorption columns during the adsorption step, the amount of the desired component that is exhausted to outside the system can be suppressed.
In case simply combining two separation methods, two adsorption columns are respectively needed in the equilibrium pressure swing adsorption process and the rate-dependent pressure swing adsorption process, that is, four adsorption columns in total.
Correspondingly, in case using the gas separation method that the gas to be separated is alternately introduced into first and second adsorption columns that are containing adsorbents having different adsorption characteristics (equilibrium type adsorbent and rate-selective adsorbent) respectively, the apparatus will be a lower capital cost.
Hereinafter, the method for separating one less readily adsorbable component from a feed gas at the first adsorption column by using an equilibrium pressure swing adsorption method and separating another less readily adsorbable component from the feed gas at the second adsorption column by using a rate-dependent pressure swing adsorption method is described.
During the adsorption steps in the first and second adsorption columns, a circulating feed gas is introduced into the adsorption column at a high pressure, and a readily adsorbable component is selectively adsorbed on the respective adsorbent, and a less readily adsorbable component is extracted as product gas.
During the regeneration steps, by depressurizing the adsorption column, the readily adsorbable component adsorbed onto the adsorbent is desorbed, and the adsorbent is regenerated. The discharge gas is mixed with a feed gas, and supplied to the adsorption column during the adsorption steps as the circulating feed gas.
In this method, as shown in Table 1, the first adsorption column is undergoing the adsorption step, while the second adsorption column is undergoing the regeneration step, and conversely, the second adsorption column is undergoing the adsorption step, while the first adsorption column is undergoing the regeneration step.
With this method, each adsorption column used for each pressure swing adsorption process can be made to be one.
TABLE 1First adsorption columnAdsorption stepRegeneration stepSecond adsorption columnRegeneration stepAdsorption step
However, since the method mentioned above cannot achieve an adequate product recovery rate, it has been desired that this recovery rate be improved. Here, the product recovery refers to the ratio of the product gas flow rate to the circulating feed gas flow rate, which is defined as follows.Product recovery rate [%]=(product gas flow rate×product gas concentration [%])/(circulating feed gas flow rate/2× concentration of product gas [%] within circulating feed gas)×100
Since the conventional method achieved a low product recovery rate, a compressor having a large capacity was needed, and there has been an issue of an increase in facility cost and operation cost.